1. Field of the Invention
The present invention relates to a method for producing a semiconductive single crystal. More particularly, the present invention relates to a method for epitaxially growing a thin film of ZnSe single crystal on a ZnSe substrate by the MOCVD method.
2. Description of the Related Art
ZnSe is one of the II/VI group compound semiconductors and one of the promising materials of blue light emitting diodes.
Hitherto, no practically usable blue light emitting diode has been produced, since there has not been produced a suitable material which has a band gap of 2.5 eV or larger, from which a large area substrate crystal can be made, with which a p-n junction can be formed and the band gap of which is the direct transition type.
Although GaN, SiC, ZnSe, ZnS, GaAlN, ZnSSe and the like have the band gap of 2.5 eV or larger, their production techniques have not been developed to such extent that the blue light emitting diode could be produced from them. Concerning the ZnSe substrate, it has been difficult to produce a substrate with high purity and low crystalline defects.
To produce an LED, a p-n junction should be formed. To this end, a p-type thin film should be formed on a n-type substrate with low resistance, or a n-type thin film should be formed on a p-type substrate with low resistance. In case of ZnSe, if a thin film of non-doped ZnSe were formed on the ZnSe single crystal substrate, the ZnSe thin film could be converted to the n-type layer by ion implantation.
To produce the LED from ZnSe, ZnSe should be epitaxially grown on the substrate. However, no bulk single crystal with good quality has been provided. Then, the ZnSe thin layer has been grown on a GaAs substrate or a Ge substrate by the MOCVD or MBE method, since such substrates can be produced with high purity and with few defects.
The epitaxial growth of ZnSe on GaAs is disclosed in many literatures, for example, W. Stutius, Appl. Phys. Lett., Vol. 38 (1981) 352 and K. Ohkawa et al, J. Appl. Phys., Vol. 62 (1987) 3216.
However, since the growth of ZnSe on the GaAs substrate is heteroepitaxy, the lattice constant is different between the ZnSe thin film and the GaAs substrate. That is, there is a mismatching of the lattice constant. Since there is a big difference in thermal expansion between room temperature and the growth temperature, large stress is generated in the thin film, which results in the formation of defects. In addition, impurities diffuse from the substrate into the formed ZnSe thin film, whereby electrical characteristics of the thin film are modified. These are the inherent drawbacks of the heteroepitaxy, and it is hardly possible to overcome these drawbacks even if the epitaxy growth is done as completely as possible. Thus, the ZnSe thin film should be homoepitaxially grown on the ZnSe substrate.
Only one paper, namely P. Blanconnier et al, J. Appl. Phys. Vol. 52, No. 11 (1981) 6895 reported the homoepitaxial growth of the ZnSe thin film on the ZnSe substrate. Blanconnier et al grew the ZnSe thin film by the MOCVD method on the (110) plane of the ZnSe substrate produced by the iodine transporting method or the Piper method.
Many proposals have been made on the growth of the ZnSe single crystal ingot. However, each of them has its own drawbacks. Since the characteristics of the substrate will have influences on the epitaxially grown thin film, the method for producing the ZnSe single crystal substrate should be taken into consideration in the growth of the ZnSe thin film on the substrate.
Though the details of the method are not clear, Blanconnier et al grew the ZnSe thin film under the following conditions:
ZnSe substrate: (110) plane of the single crystal produced by the Piper method or the ion transporting method PA0 Zn source: Diethylzinc [Zn(C.sub.2 H.sub.5).sub.2 ] PA0 Se source: H.sub.2 Se PA0 Substrate temp. 500.degree. C. PA0 Growth rate 100 .ANG./min. PA0 Thickness: 1-2 .mu.m PA0 Dislocation density in the epitaxial film: 10.sup.6 /cm.sup.2
On measuring such thin film with the photoluminescence (4.2 K.), strong emission spectrum due to excitons constrained with neutral donors appeared.
The ZnSe thin film grown on the substrate produced by the Piper method contains Ga as the impurity, which might be contained in dimethylzinc. The ZnSe thin film grown on the substrate produced by the iodine transporting method contains iodine atoms as the impurity, which might migrate from the substrate since the substrate contains the iodine atoms.
Blanconnier et al reported that many hillocks were present on the surface of the epitaxially grown thin film by the observation with a scanning type electron microscope. Such hillocks are assumed to be defects caused by the dislocation in the substrate.